"In 1011, Rudolph III of Burgundy presented a Novum castellum or new castle
(Old French neu, now neuf and Old French châtel, now château, in antiquated
German: Welschen Nüwenburg or Newenburg am See) on the lake shore to his wife
Irmengarde. It was long assumed that this new castle replaced an older one, but
nothing about its location or design is known. At the time of this gift
Neuchâtel was probably the center of a newly created royal court, which was
recently developed to complement the other royal estates which managed western
estates of the Kings of Burgundy". (This was the official starting point of the
commune (ville), thus celebrating in 2011 its 1,000 years of establishment).
Source:
http://en.wikipedia.org/wiki/Neuchatel#Middle_Ages

They have two commemorative flags to celebrate the 1,000 years. They are
both horizontal flags, they show the year 2011 (split showing the upper half of
the writing, on top of "Neuchâtel / A mille ans" in upper case letters. One
version is red background with white letters, the other version having both
colors inverted.